Implantable medical devices (IMDs) detect and deliver therapy to correct a variety of medical conditions in patients. Exemplary IMDs include implantable pulse generators (IPGs) or implantable cardioverter-defibrillators (ICDs) that deliver electrical stimulation to tissue of a patient. IMDs typically include, inter alia, a control module, a capacitor, and a battery. These components are housed in a container that is hermetically sealed to prevent liquid from contacting the electronic components therein. To connect with the electronic components, a feed-through assembly is coupled to the container. The feed-through assembly forms an aperture in the container, which allows an electrical lead to pass therethrough.
Electromagnetic interference (EMI) may affect the operation of an IMD. EMI is any electromagnetic disturbance, phenomenon, signal, or emission that causes or can cause an undesired response in an IMD. To address EMI, an EMI filter is incorporated into a feed-through assembly. A typical filtered feed-through assembly consists of a conductive line, a ferrule, an insulator member (e.g. glass, ceramic etc.), at least one capacitor, and a seal. The filtered feed-through assembly is configured such that the seal lines an aperture located in the ferrule. The insulator member is placed in the aperture adjacent to the seal. The conductive line is connected to the ferrule and the capacitor. A lead (e.g. wire) or a terminal pin to the sealed container is inserted through another aperture in the insulator member, which provides an electrical connection to the components within the IMD.
The filtered feed-through assembly capacitor acts as a low pass filter to prevent EMI from affecting the operation of the IMD. A low pass filter allows low frequency signals to pass but prevents high frequency signals from passing therethrough. The performance of the EMI filter is determined by insertion loss. Insertion loss results from the insertion of a device in a transmission line, expressed as the reciprocal of the ratio of the signal power delivered to that part of the line following the device to the signal power delivered to that same part before insertion. Insertion loss depends upon the number of components in the EMI filter, impedance value of each EMI filter component, the frequency at which the insertion loss is measured, equivalent series resistance (ESR), and equivalent series induction (ESL).
Insertion loss measurement at high frequencies is difficult due to increased noise detected for high frequency signals. To determine insertion loss at a high frequency, a radio frequency (RF) shield is welded to the filtered feed-through assembly. The RF shield isolates the input and output ends of the filtered feed-through assembly during the test. The welding operation and the RF shield itself increase the cost of producing an IMD. Additionally, this insertion loss measurement method cannot be used on a large-scale basis.
Alternatively, insertion loss may be measured without a RF shield welded to the filtered feed-through assembly. However, this type of insertion loss measurement is unreliable. It is therefore desirable to overcome the limitations associated with conventional testing systems.